1. Field of the Invention
The present invention relates to a magneto-optical recording and reproducing apparatus and, more particularly, to an improvement in the high-density magneto-optical recording means of a magneto-optical recording and reproducing apparatus of a magnetic field modulation system.
2. Description of the Related Art
In a magneto-optical recording and reproducing apparatus, an information recording signal is recorded by varying the direction of magnetization of a magneto-optical recording thin film which is made of Gd.multidot.TbFe, Gd.multidot.Co or other similar material by applying a magnetic field and semiconductor laser light to the magnetic thin film. In reproducing a recorded signal, the magnetic thin film is irradiated with linearly polarized laser light, and the recorded signal is read out by utilizing the rotation of the plane of polarization of the linearly polarized light in accordance with the direction of magnetization due to the Kerr magneto-optical effect or the Faraday effect. The Kerr magneto-optical effect refers to the tendency of the light reflected by the surface of a recording medium to rotate, and the Faraday effect refers to the tendency of the light passing through a recording medium to rotate.
A magneto-optical recording and reproducing apparatus enables not only rewriting by way of erasure or overwriting like a magnetic recording and reproducing apparatus but also writing and reproduction at a high speed. In addition, a magneto-optical recording and reproducing apparatus has a memory with a large capacity. Therefore, much is expected of a magneto-optical recording and reproducing apparatus as an apparatus for recording and reproducing picture and other information.
The following two methods are conventionally known as a system for writing a recording signal in a magneto-optical recording and reproducing apparatus.
A first recording system is what is called a light modulation system. This is a system for recording information by irradiating the surface of a recording medium consisting of a magnetic thin film with laser light which is modulated in accordance with a recording signal. In the first recording system, namely, the light modulation system, the magnetization sensitivity of a recording medium is generally enhanced by the application of an unmodulated magnetic field to the surface of the recording medium simultaneously with irradiation by laser light, and it is possible to record with comparatively weak laser light.
A second recording system called a magnetic field modulation system. This is a system for recording information by irradiating the surface of a recording medium consisting of a magnetic thin film (magneto-optical recording film) with unmodulated laser light and simultaneously applying a modulated magnetic field thereto in accordance with a recording signal. In the second recording system, namely, the magnetic field modulation system, information is recorded only at the portion which is irradiated with laser light because the magnetic field applied to the surface of the recording medium is so weak that recording with its own field strength is impossible. The magnetic field is generated by applying a current corresponding to the recording signal to the wire in the vicinity of a recording portion.
FIG. 1 shows the functional structure of a conventional high-density magneto-optical recording and reproducing apparatus of a magnetic field modulation system.
This apparatus is composed of a laser light source 10 for generating semiconductor laser light, a laser driver 12 for driving the laser light source 10, and a signal line L1 for supplying a signal to the laser driver 12. The laser light emitted from the laser light source 10 is controlled in accordance with a control signal which is supplied to the laser driver 12 through the signal line L1.
The apparatus is further composed of a reflecting mirror 14 for changing the optical path of the laser light emitted from the laser light source 10, an optical head 15 having an objective lens 15a for converging the laser light into the form of a spot, a driving motor 17 for driving a record carrier 16 at a predetermined rotational speed, and a beam splitter 18 disposed between the reflecting mirror 14 and the laser light source 10. The beam splitter 18 has the function of splitting the laser beam reflected by the record carrier 16. The reflected light which has passed through the beam splitter 18 is received by a reproducing optical system 19, wherein the focusing state, the tracking state and a reproducing signal are detected. The reproducing optical system 19 is composed of, for example, a condenser lens, a wave plate (polarizing plate) and a light receiving element (photoelectric conversion element) such as a CCD.
A magnetic field applying means for applying a magnetic field, which has its polarity inverted or strength modulated in accordance with recording information, to the record carrier 16 including a magneto-optical recording film, is composed of a magnetic field generating coil 20 for applying a magnetic field to the record carrier 16 at the time of irradiation by a laser beam, a coil driver 21 for driving the magnetic field generating coil 20, and a signal line L2 for supplying a recording signal to the coil driver 21.
In the conventional apparatus having the above-described structure, the magnetic field of the coil 20 is modulated in accordance with a recording signal which is supplied to the coil driver 21 through the signal line L2, and the recording signal is recorded in the record carrier 16 by a laser beam which is controlled in accordance with the control signal which is supplied through the signal line L1.
The record carrier 16 including the magneto-optical recording film is irradiated with the laser beam which is converged in the shape of a spot by the objective lens 15a, and at the same time the magnetic field which is generated by the magnetic field generating coil 20 and modulated in accordance with the recording signal supplied through the signal line L2 is applied to the record carrier 16.
Although the laser beam with which the record carrier 16 is irradiated is controlled in accordance with the control signal which is supplied to the laser driver 12 through the signal line L1, it is not modulated. On the other hand, the magnetic field applied to the record carrier 16 by the magnetic field generating coil 20 is modulated in accordance with a recording signal which is supplied to the coil driver 21 for driving the magnetic field generating coil 20.
Information is therefore recorded in the record carrier 16 by modulating the magnetic field which is applied to the record carrier 16. The strength of the magnetic field which is applied to the surface of the recording medium at the time of recording is so weak that recording with its own field strength is impossible. Information is therefore recorded only at the portion which is irradiated with the laser beam.
In order to enhance the recording density of such a magneto-optical recording and reproducing apparatus of a magnetic field modulation system, it is necessary to reduce the diameter of the light spot which is used for recording and reproduction.
In overwriting in a conventional magnetic field modulation system which directly uses semiconductor laser light, the reduction in light spot diameter which is set when the light spot is formed on the surface of a recording medium by semiconductor laser light is greatly restricted by the semiconductor laser oscillation wavelength. For example, in an AlGaIn semiconductor laser, which has been reported to oscillate light at the shortest wavelength, it is said to be theoretically possible to obtain a semiconductor laser oscillation wavelength of 580 nm (0.58 .mu.m).
The light spot diameter .omega. is generally obtained from the formula for obtaining the first dark ring of the Airy disk as follows: EQU .omega.=1.22.times..lambda./NA
wherein .lambda. represents the wavelength of the light source, and NA the aperture of the objective lens. If the aperture NA of the objective lens is 0.6, for example, the light spot diameter .omega. obtained by using the above-described semiconductor laser light having a wavelength of 580 nm (0.58 .mu.m) is calculated from the above-described Airy's formula as follows: EQU .omega.=1.22.times..lambda./NA=1.18 .mu.m.
The thus-obtained diameter 1.18 .mu.m is the minimum light spot diameter in practical use. However, the light spot diameter of 1.18 .mu.m cannot be said to be adequately small for enhancing the recording density of a magneto-optical recording and reproducing apparatus and increasing the data transferring speed by reducing the recording bit length. It is therefore necessary to produce a magneto-optical recording and reproducing apparatus which is capable of magneto-optical recording and reproduction with a smaller light spot diameter.